1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a reflection type liquid crystal display device preferably used in office automation appliances such as word processor and notebook type personal computer, various video appliances, and game appliances.
2. Description of the Related Art
The liquid crystal display device is used presently in wide fields including clock, pocket calculator, computer terminal, word processor and TV set. The representative display mode used in such applications is the so-called TN (twisted nematic mode) which twists the liquid crystal molecules in the liquid crystal display element nearly by 90 degrees as the initial orientation.
The TN mode is intended to display monochromatically by disposing liquid crystal display elements between a set of polarizers and by making use of the optical properties of the liquid crystal elements, that is, the optical rotatory characteristic when voltage is not applied, and the canceling optical rotatory characteristic when voltage is applied.
For color display, color filters of, for example, red, blue and green are disposed in the liquid crystal display elements, and by making use of the light switching properties in the TN mode, multi-color display or full-color display is realized by additive color mixture. This color display method is presently used in the display of a portable so-called pocket liquid crystal display television applying the active matrix drive or simple matrix drive.
The display mode widely used in the display for word processor is the STN (super twisted nematic mode) having the liquid crystal display element structure similar to that of the TN mode, in which the twisting angle of liquid crystal molecules is set at 180 to 270 degrees. It is the feature of the STN mode that an electro-optical characteristic possessing a sharp threshold value is realized by reflecting the sudden molecular orientation deformation due to increase of applied voltage in the double refraction (birefringence) change of liquid crystal by the increase of twisting angle of liquid crystal molecules over 90 degrees and optimization of the polarizer setting angle. It is therefore suited to simple matrix drive.
On the other hand, in this STN mode, the background color of the display is tinted with yellow-green or dark blue color due to double refraction of liquid crystal. As an improvement, it is proposed to use a liquid crystal display element capable of displaying monochromatically by compensating the colors by laminating the STN panel for display with an optical compensating panel or phase differential plate formed of high molecules of polycarbonate or the like. At the present, this liquid crystal display element structure is available on market in the name of paper white LCD. As the color display in STN mode, multicolor display or full-color display is realized in the same principle of operation as in the TN mode.
Besides, where a wide viewing angle is required, the so-called GH (guest-host) mode Is used, in which pigments differing in the light absorbance in the major axis (molecule axis) direction and minor axis direction (dichroic pigment) are added to liquid crystal molecules. The GH mode is classified into the Heilmeier type using polarizer, White-Taylor type (phase transition type) not using polarizer, and two-layer type, and regardless of the type the principle of operation is to control the orientation of the pigments through orientation of the liquid crystal molecules by voltage and display by making use of the difference in light absorbance in the pigment molecule direction. The color display is realized by using pigments that absorb the wavelength of a part of visible light, or combining the GH mode liquid crystal display element using black pigment with colored filters. The details of principle of operation of the TN mode, STN mode and GH mode are described in "Liquid Crystal Device Handbook" (edited by Committee 142 of Japan Society of Scientific Promotion, pp. 315-346, 1989).
In the display mode using polarizers, the light utility rate is decreased at least to less than 50% depending on the optical properties of the polarizers. Therefore, in order to realize a, so to speak, bright display demanded in the reflection type display or projection type display, the display mode without polarizer is advantageous. From this point of view, possible candidates among the known display modes are the White-Taylor type (phase transition type) GH mode and two-layer type GH mode.
Comparing these two display modes, it is already reported that the two-layer type GH mode is superior in the contrast and brightness (Proc. of the SID. 2514, 1984, p. 275). In the liquid crystal display element of the conventional two-layer type GH mode, however, since an ordinary glass substrate is used between liquid crystal layers, parallax is caused between the upper and lower liquid crystal layers depending on the viewing angle, and the display is blurry.
As its improving measure, it may be considered to use a very thin glass sheet between the liquid crystal layers, but it is difficult in handling and difficult to control the thickness of the liquid crystal display elements. It is therefore not applicable to the display of high precision in a relatively wide area.
Concerning the problem of parallax, the White-Taylor type GH mode is advantageous because it is composed of one layer, but its display precision is inferior as stated above, and in the case of multicolor display, in particular, since the microcolor filter is needed, the brightness is lost, and the display precision is inferior.